This invention relates to improvement in a dual system hydraulic brake device for use in vehicles.
With a hydraulic brake device for use in vehicles, it is a common practice to use a dual system type brake device to cope with loss of braking function in the event of troubles such as due to oil leakage in either of the brake systems.
For clarifying the disadvantages of the prior art dual system hydraulic brake device, we wish to refer to FIG. 1 which shows a schematic view of a conventional dual system hydraulic brake device. An outlet of a pump 1 producing a hydraulic pressure is connected by way of a pipe 2 to inlets of two check valves 3 and 4. The pipe 2 is also connected to an inlet of a relief valve 5, the outlet of which is connected by way of pipe 6 to a reservoir 7. The outlets of the check valves 3 and 4 are connected by way of pipes 8 and 9 to respective inlets of a dual system control valve 10 as well as connected by way of branched pipes 11 and 12 to hydraulic accumulators 13 and 14. The respective outlets of the control valve 10 are connected by way of pipes 15 and 16 to a brake circuit 17 for system I and to a brake circuit 18 for system II, respectively. Shown at 19 is a brake pedal which is adapted to force pistons 21 and 22 into the control valve 10 by way of a push rod 20. The outlets for return oil, of the control valve 10 is connected to the reservoir 7 by way of pipes 23 and 24 and a pipe 25 which is commonly connected to pipes 23 and 24 as well as to the inlet of the pump 1. With a conventional system of such an arrangement, when a driver treads down the brake pedal 19, the push rod 20 will force pistons 21 and 22 such that the hydraulic pressure, which is being supplied from the pump 1 or from the accumulators 13 and 14 to the control valve 10, is applied to the brake circuit 17 for the system I and the brake circuit 18 for the system II simultaneously, thereby effecting a braking operation.
In the event of failure in either of the systems, for example, system I, such as due to oil leakage, hydraulic pressure of the pump 1 can not be utilized in system II, since the check valve 3 will be opened at a relatively low pressure. As a result, braking operation in the system II will be achieved only by means of hydraulic fluid stored in the accumulator 14. While, an accumulator equipped on an automobile is usually capable of 10 to 20 times braking operations without receiving hydraulic fluid from a pump. Accordingly, in the event of failure in one of the systems, the other system will soon become inoperable.
One of the solutions to avoid this shortcoming is to provide two systems including pumps, which are entirely independent of each other. However, this necessarily leads to a complicated construction, with the increase in number of parts and cost, in addition to added power loss caused due to the use of two pumps, thus failing to meet desired practical use.